The present invention relates generally to polymer foam processing and, more particularly, to a system and method of metering blowing agent in a polymeric foam process.
Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. A number of techniques for processing polymeric material utilize an extruder which plasticates polymeric material by the rotation of a screw within a barrel. Certain polymeric foam processes involve injecting a physical blowing agent into molten polymeric material, for example, through a blowing agent port formed within the barrel to form a mixture of polymeric material and blowing agent. The mixture may be processed (e.g., extruded, blow molded or injection molded) to form the desired polymeric foam article.
Blowing agent injection systems include a blowing agent source which may be fluidly connected to the blowing agent port. Such injection systems may also include one or more valves to control blowing agent flow, pumps to pressurize the blowing agent, and other devices such as pressure regulators. In some cases, the injection system may also function as a metering system that controls the rate of introduction of blowing agent into the polymeric material in the extruder barrel. Such metering systems may be used, in particular, in processes that require relatively precise control over blowing agent introduction, such as certain processes for producing microcellular materials. Microcellular materials are polymeric foams that have small cell sizes and large cell densities.
Some conventional metering systems include a flow meter, a metering valve, and a controller. During operation, the controller adjusts the metering valve to provide a desired flow rate in response to inputs from the meter and, in some cases, inputs from an operator. Such metering systems are generally expensive, in part, due to the cost of flow meters. The accuracy of such metering systems may also be compromised by fluctuation in pressures within the extruder.
Accordingly, there is a need for blowing agent metering systems that accurately meter blowing agent and are relatively inexpensive.
The invention provides a blowing agent metering system and method for use in polymeric foam processing. The system may be used to control the rate of introduction of blowing agent into polymeric material in a processing apparatus to form a mixture having the desired blowing agent weight percentage. The metering system includes a restriction orifice which is positioned in a pathway between a blowing agent source and a blowing agent port within a foam processing apparatus. The metering system measures the pressure differential as blowing agent flows across the restriction orifice from the source to the port and may also measure the temperature of the blowing agent at one or more locations. Using a pre-determined relationship between the flow rate, the orifice dimensions (e.g., length, diameter) and the measured variables (e.g., pressure differential, temperature), a controller of the system adjusts the pressure upstream of the restriction orifice in response to inputs of the measured variables to maintain a pressure differential across the orifice that provides the desired blowing agent flow rate.
In one aspect, the invention provides a blowing agent metering system. The metering system includes a blowing agent delivery conduit connectable to a source of blowing agent and to a blowing agent port formed within a polymer processing apparatus and able to deliver blowing agent from the source to polymeric material within the processing apparatus. The conduit includes a section defining a restriction orifice. The metering system further includes a pressure differential measuring device constructed and arranged to measure the blowing agent pressure differential across the restriction orifice. The metering system further includes an upstream pressure regulating device associated with the conduit upstream of the restriction orifice.
In another aspect, the invention provides a polymer processing system. The polymer processing system includes an extruder including a polymer processing screw constructed and arranged to rotate within a barrel to convey polymeric material in a polymer processing space defined between the screw and the barrel in a downstream direction. The barrel has a blowing agent port formed therein. The system further includes a blowing agent delivery system including a conduit connectable to a source of blowing agent and to the blowing agent port. The conduit includes a section defining a restriction orifice. The blowing agent delivery system includes a pressure differential measuring device constructed and arranged to measure the blowing agent pressure differential across the restriction orifice, and an upstream pressure regulating device associated with the conduit upstream of the restriction orifice.
In another aspect, the invention provides a method of metering blowing agent. The method includes metering introduction of blowing agent into polymeric material in a polymer processing apparatus by regulating pressure of blowing agent upstream of an orifice through which blowing agent passes in response to inputs of the blowing agent pressure differential across the orifice.
Among other advantages, the blowing agent metering system may be used to accurately meter the flow of blowing agent into polymeric material over a wide range of blowing agent flow rates. The metering system may regulate in real time the pressure differential across the orifice to provide the desired flow rate so that system can account for pressure fluctuations that may exist in the extruder throughout the process. Furthermore, the blowing agent metering system has a simple design and may be manufactured relatively inexpensively. Also, the blowing agent delivery system may be used with any polymer processing system including extrusion, injection molding and blow molding systems.